Beam recorders and flying spot scanners having continuous motion film drives are generally used either for recording television images on motion picture film in the case of a recorder, or for reading out an image, in the case of a flying spot scanner. Each frame of the film is scanned in a line-by-line fashion by means of an electron beam, light beam, or laser beam, which is caused to sweep horizontally across the film as it moves through the film guide. Vertical positioning of the beam is provided mainly by the motion of the film itself as it passes through the scanning area of the film guide at a constant velocity. A continuous-motion film drive is needed when (as in real-time recording) there is insufficient vertical blanking time to allow pull-down of the film between frames.
The difficulty with continuous motion film drives derives from the fact that the film is edge guided during recording or readout. In the case where the film was recorded with such a device, for example, when the film is subsequently developed and printed, a pin registered movement is used. The pin registered movement uses the sprocket holes rather than the edge of the film to position the recorded frames. It has been found that the edge of the film has a tendency to vary in its distance with respect to adjacent sprocket holes, due to manufacturing tolerances in the making of the film. Therefore, when viewing the resultant film, the picture seems to horizontally shift position, i.e. weave, in a random manner. A similar effect is seen if the film was initially recorded using a pin registered movement, and a scanner having a continuous motion film drive is used to reproduce the film images.
An earlier attempted solution to this weaving problem generated in continuous motion film drive scanners was to add a sprocket drive to the continuous motion film drive. Such a drive was added at the point where the picture was scanned, to provide correct horizontal positioning of the film sprocket holes with respect to the beam. However, this method introduced small but abrupt speed changes as the sprocket tines entered the sprocket holes. This caused random lines of the television picture to separate during recording, which in turn caused unacceptable random horizontal black lines in the resultant recorded picture.
A second solution was to mechanically measure the position of the sprocket holes during the recording or reading of images on the film, and to use this position measurement to adjust the horizontal deflection of the beam. This method has been attempted, but without much success.